There are many different potential causes for joint pain and these include infection, accidents, crystals in the joint, inflammation and articular cartilage damage. The clinical history and joint examination are the strongest guides to establishing a differential diagnosis and once a provisional decision about the likely cause has been made then laboratory investigations can be useful to confirm or refute this. Without an initial guiding thought as to the possible diagnosis investigations can be more confusing than enlightening. The primary effort is in pinning down the likely pain source and then working out the disease process.
The source of joint pain can be from within the obvious joint, from structures which surround the joint or can be a referred pain from a structure elsewhere. Within the joint itself the pain can come from the surrounding capsule, the ligaments, the bone membrane, the synovial joint lining and the bone underneath the joint cartilage. The joint cartilage itself is not a source of pain as it does not possess any sensory nerve endings. It is very important to pin down the likely structure responsible for the pain as the diagnosis and so the therapy may be different depending on that evaluation.
The pathological processes which result in joint damage are several and cover infections, laying down of crystals in the joint, inflammation of the junctions between tendon and bone and ligament and bone (enthesitis), synovial inflammation and joint abnormalities. Joint changes can be structural or mechanical like injury to a meniscus. The above pathologies may occur in combination and not just one at a time. Inflammatory changes in the joint linings (synovitis) are the main pathology in many arthritic conditions including rheumatoid arthritis. The growth of the synovial membrane can cause cartilage destruction and a puffy, warm and inflamed joint.
The enthesis is the area where a tendon or a ligament inserts into the bone and this area is preferentially affected in some types of joint condition. Inflammation of these areas causes the collagenous soft tissue close to the bone to form bone which grows out from the origin either along the ligaments or the outer disc layers. If crystals are deposited in a joint this can lead to an inflamed and painful joint, with crystals forming on the cartilage itself, on the synovium or on the structures around the joint. Localised or generalised symptoms can result and a gouty joint is recognisable by the warmth, redness and extremely sharp pain on stressing or moving.
An infectious arthritis may be caused by bacteria, viruses or fungi and is typically carried by organisms borne in the blood. The person may be unwell as this can be part of a system wide infection. Infections occur in the synovial membranes with some death of the membrane and significant formation of healing tissue and scar. Bacteria are capable of releasing toxins which have the ability to cause rapid destruction of cartilage. In structural abnormalities of the joint osteoarthritis is the commonest joint pathology in the world, with both internal and external factors affecting its incidence and severity.
Osteoarthritis is more likely to occur in a joint which has sustained previous trauma such as a fracture or a meniscal tear, suffers from a congenital structural abnormality or suffers from an abnormality of collagen such as hypermobility. A person’s genetic inheritance, their level of obesity and their occupation may also be relevant, although one of the strongest correlating factors is age. The deterioration of the joint surfaces in osteoarthritis is accompanied by an increase in the underlying bone’s density and by the growth of bony spurs at the margins of the joints. A synovitis may occur but often osteoarthritic change does not seem to involve inflammation.
The clinician who examines the patient needs to decide the likely source of the pain. It may be that the joint truly is the pain source but other nearby areas such as tendons, ligaments and muscles may be responsible or a distant pain source may refer pain to the area. The shoulder and hip, joints closer to the centre of the body, are harder to diagnose in this way. Pain in the hip region can be due to lumbar stenotic change, degenerative disc disease, osteoarthritis or bursitis laterally.
